The invention has an industrial application in machines intended for thermoforming, by means of blowing, plastic containers which are intended for filling, in a sterile environment, with food or cosmetic products or for other uses and which, for these purposes, are blown with sterile air and emerge from the thermoforming moulds with a hermetically sealed, closed, end portion which is removed during filling of the containers performed in a controlled-atmosphere environment.
In order to prevent these containers from being deformed during the cooling step following the thermoforming step, before closing of the said end portion and extraction of the blowing nozzle from the latter, it is necessary to achieve inside the said containers a vacuum value which balances the increase in internal pressure caused by the residual heat of the plastic which forms the said containers.
In the prior art, the formation of the correct vacuum inside the containers thermoformed by means of blowing is achieved using costly apparatus which do not take into account the atmospheric pressure of the environment in which the said containers are produced and which, upon reaching the set vacuum value, isolate the container from the connection with the suction circuit, while the means which transversely close the said end portion of the said container are operated in synchronism. Small variations in the relative times of the abovementioned steps influence the value of the vacuum reached inside the containers and may result in production rejects.
The invention intends to overcome the limitations of the prior art with the following solution. When forming of the containers has been completed, the blowing nozzles, which are still sealingly engaged inside the said containers, are isolated from the blowing circuit and are connected to a discharge circuit connected to the suction mouth of a Venturi meter or ejector operated by an air stream, the pressure of which is modulated by an electronically controlled proportional valve. With this apparatus it is possible to create inside the containers the desired vacuum value, with a high degree of precision which may be maintained over time, such that the means for closing the end portion of the said containers are able to operate with any time variation from the instant the predefined vacuum value is reached, without production rejects arising, as instead occurs in the prior art.
The electronic control unit which controls the proportional pressure valve of the primary circuit of the ejector also takes into account the value of the atmospheric pressure of the external environment in which the containers are produced, so that the value of the vacuum created inside the thermoformed containers is determined in proportion to the value of the said atmospheric pressure. In other words, the value of the vacuum inside the containers will be corrected positively or negatively with respect to a reference value, depending on whether the atmospheric pressure varies more or less with respect to a predefined mean value.
Further characteristic features of the invention and the advantages arising therefrom will emerge more clearly from the following description of a preferred embodiment thereof, illustrated solely by way of a non-limiting example in the figures of the accompanying plates of drawings, in which:
FIGS. 1 and 2 illustrate the general diagram of the apparatus in the thermoforming and depressurization condition of the containers, respectively;
FIG. 3 shows a side elevation view of a practical embodiment of the invention;
FIG. 4 shows the graph for variation in the internal vacuum of the containers in relation to the atmospheric pressure of the working environment.